Liquid scintillation counting is a generally known and widely used technique for the measurement of low energy beta emitting radionuclides in addition to gamma and alpha emitters. The liquid scintillation counter is utilized to make quantitative measurements of radioactivity when incorporating a radio-labeled analyte into solution with a liquid medium capable of producing photons resulting from the kinetic interactions of nuclear decay products. There are different radionuclides, both man-made and naturally occurring, which can be experimentally employed to illustrate the molecular dynamics of a particular model by measuring their distributions in various systems. It is often desirable to use multiple radionuclides in the same experiment to follow different distributions in the same system simultaneously.
The energy of the beta particles ranges from zero energy to the characteristic end point maximum energy for that radionuclide. A pulse height energy distribution spectrum may be recorded for the solution being analyzed. When the solution being analyzed contains a mixture of beta emitters there is an overlap of the spectra from each emitter. The area under a pulse height energy distribution spectrum increases as decay events are accumulated over a period of time corresponding to the energy of the decay event. The total area of this spectrum may be correlated to the Spectral Index of the Sample (SIS) or alternatively the measured end point maximum energy.
Specific radionuclides have developed immense importance in various applications wherein radio-labeled components are utilized. By dissolving a sample containing the radionuclide in a mixture with a typical liquid scintillation cocktail, such as an aromatic solvent containing an organic scintillator, energy from the nuclear decay is transferred with reasonable efficiency to the scintillators which emit multiple photons proportional to energy of the beta particle. The light is detected by sensitive photomultiplier tubes. In a normal state these photomultiplier tubes have a characteristic dark noise which occurs upon the spontaneous emission of an electron within the tube. The dark noise consists of pulses generating a low intensity photon response. Thus, by utilizing multiple photomultiplier tubes and by requiring a coincidental event, beta emitters can be measured while minimizing the background. The higher the energy of the beta particle, the more photons will be produced over selected time intervals such as 20-50 nanoseconds. All photons produced within that interval are considered a pulse. The combined amplitude of the photons is known as the pulse height. The range of pulse heights is a smooth distribution of energies ranging from zero energy rising to a maximum and tailing off to the maximum energy characteristic of the beta emitting radionuclide. This distribution is known as the pulse-height energy distribution spectrum.
Another phenomenon of interest to a liquid scintillation user is the phenomenon of quenching. All energy created by the beta particle in reaction with the fluor is unfortunately not dissipated into the production of light. The adding of sample material to the scintillation solution introduces molecules which absorb energy and which may not be capable of producing light. A portion of the energy of the ejected decayed particles is dissipated in the excitation of solvent molecules as well as by the indirect transfer of energy to obstructive sample molecules otherwise not capable of energy transfer or the conversion of that energy into photons. Since some of the energy transferred to sample molecules is not utilized to produce photons, it is not measured by the photomultiplier tubes and not recorded. Quenching results in the spectrum of pulse amplitudes having an apparent diminished energy when compared to the theoretical spectrum for an unquenched sample.
The relative quenching of the sample may be determined by using an external source quench-indicating parameter (QIP) such as the Spectral Index of the External Standard (SIE). This method involves counting the sample with a gamma radiation source adjacent the vial and subsequently in the absence of the gamma radiation source such that the contribution of only the gamma radiation source is studied. The gamma radiation source generates compton electrons in the sample solution which behave in a similar manner to decaying nuclear particles. If quenching is present the pulse-height energy distribution spectra from the gamma radiation generated events will be compressed towards a lower apparent energy. External standard quench-indicating parameters (QIP) include the external standard ratio (ESR), Spectral Index of the External Standard (SIE); H Number, ESP and the transformed Spectral Index of External Standard (tSIE).
It has been determined that a correlation equation which is essentially a straight line exists for the comparison of an external standard quench-indicating parameter and a quench-indicating parameter of a sample when both are measured at varying degrees of quenching. This equation is of the form of y=mx+b. Samples of various radionuclides and the appropriate radionuclide equation for each as determined on a Packard Instrument Company, Inc. Model 2000CA liquid scintillation counter are:
______________________________________ Tritium SIS = 0.015tSIE + 3.604 Carbon-14 SIS = 0.164tSIE + 3.359 Chlorine-36 SIS = 1.043tSIE + 14.969 ______________________________________
As may be seen from these radionuclide equations each defines a straight line of a slope m (0.015 for Tritium, 0.164 for Carbon-14 and 1.043 for Chlorine-36) which intercepts the SIS axis of a plot of SIS versus tSIE at the value b (3.609 for Tritium, 3.359 for Carbon-14 and 14.964 for Chlorine-36).
It can be seen that the slope of the equation increases with the molecular mass of the radionuclide which is the equivalent of the slope of the equation increasing with the radionuclide energy maximum end point or with the area under the sample spectrum distribution. This slope varies from almost no slope for Tritium to almost a vertical line for radionuclides with higher energy maximums. Each radionuclide has a distinct radionuclide equation and slope.
Since the radionuclide equation for each radionuclide is different, once a table of radionuclide equations is established, then the appropriate combination of a quench-indicating parameter of the sample such as SIS or the measured end point maximum energy and the external source quench-indicating parameter such as tSIE for a sample can only satisfy one of these equations. Once this equation is identified, the radionuclide is thereby identified. This sample need only be tested at a single quench level to determine if the values satisfy a specific radionuclide equation.
If a sample is tested at different quench levels, then the slope of the radionuclide equation for the sample can be calculated. This sample slope can then be compared only to the slope portion of each radionuclide equation to determine a match. Again, once the slope is identified to be that of a particular radionuclide, the identity of the radionuclide is known.
If no match is obtained between the test sample and the known radionuclide equation, it may be an indication of one or more problems in the test arrangement. If the sample is nonhomogeneous or contains radionuclides not having an equation in the look-up table, then no match should be achieved. Equipment malfunctions and other sample preparation errors could likewise result in identification of a radionuclide not being made.
In the event the sample contains a multiplicity of unknown radionuclides, the measured end point maximum energy for the highest energy emitter of the combinations can be used to identify this radionuclide. The radionuclide equation in this case would be based on the end point maximum energy representing the sample QIP as a function of tSIE for a given radionuclide at different degrees of quenching. For a full explanation of using the end point maximum energy as a quench-indicating parameter see U.S. Pat. No. 4,633,088 entitled "Reverse Sum Quench Measurement Using A Liquid Scintillation Counter".
The herein invention utilizes regionless counting of the disintegrations per minute of the radionuclides. The invention is directed towards determining the Spectral Index of the Sample (SIS) or first moment of the spectrum or mean pulse height or some other quench-indicating parameter (QIP) of the sample. A QIP based on an external standard such as the transformed Spectral Index of the External Standard (tSIE) of the radionuclide in the mixture is then determined for the sample mixture using regionless counting.
An advantage of utilizing regionless counting is that all the spectral information is used to determine the quench-indicating parameters and to make the desired calculations. Overall theoretical uncertainty using the total spectral information is less than theoretical uncertainty using discrete regions.
Additionally, an advantage of this scheme is that standards necessary to develop appropriate radionuclide equations may be done once and the appropriate information stored in a machine as opposed to being done in each instance for each particular radionuclide. Additionally the counting uncertainties associated with the discrete multiple energy region counting are reduced. Theoretically, more accurate measurements can be made than existing techniques since the entire spectral value information is utilized to reach the appropriate conclusion. This technique may be used to automatically identify or verify the radionuclide being tested in each instance. If used automatically the liquid scintillation counter may indicate to the operator the presence of a problem should the wrong radionuclide be identified or should no radionuclide be identified.